1. The Field of the Invention
The present invention relates generally to surgical devices and methods for fusing adjacent bone structures and, more specifically, to surgical devices and methods for fusing adjacent vertebrae.
2. The Relevant Technology
The spinal column is made up of thirty-three vertebra each separated by a cushioning disc. Disease and trauma can damage these discs, creating instability that leads to loss of function and excruciating pain. Spinal fusion implants provide a successful surgical outcome by replacing the damaged disc and restoring the spacing between the vertebra, eliminating the instability and removing the pressure on neurological elements that cause pain. The fusion is accomplished by providing an implant which recreates the natural intervertebral spacing and which has an internal cavity with outwardly extending openings. The internal cavity is commonly filled with osteogenic substances, such as autogenous bone graft or bone allograft, to cause the rapid growth of a bony column through the openings of the implant.
Recently, adjustable fusion implants have been developed that allow the surgeon to adjust the height of the implant. This provides an ability to intra-operatively tailor the implant height to match the natural spacing between the vertebrae. This reduces the number of sizes that the hospital must keep on hand to match the variable anatomy of the patients. However, the prior art is replete with adjustable fusion implants that have an active mechanism for expanding the implant to change its height. Active mechanism refers to a mechanical structure built into the implant to cause the change in the height dimension. The presence of the active mechanism significantly decreases the amount of internal space available for placement of bone graft and other osteogenic substances to encourage the bony fusion between the adjacent vertebrae. It would therefore be an improvement over the prior art to provide an adjustable fusion implant that does not require the presence of an active mechanism, thereby maximizing the internal space for osteogenic substances and providing a better inducement for bony fusion.
Other adjustable fusion implants known in the art are comprised of modular components that must be pre-assembled prior to implantation. It would therefore be an advantage to provide a fusion implant that can be adjusted in situ.
Another challenge associated with spinal fusion is the restoration of the curvature of the spine. This curvature is present at each intervertebral level at varying degrees, and is manifested by a different spacing or height at the anterior and posterior margins of adjacent vertebral bodies. For example, the lumbar spine has a natural curvature when viewed from a lateral perspective referred to as lordosis, where the mid section of the lumbar spine is more anterior than the end sections. Thus, at any given intervertebral level, the intervertebral height at the posterior margin is less than the intervertebral height at the anterior margin, resulting in a wedge shaped disc or intervertebral space.
When a spinal fusion implant is placed from the posterior aspect of the vertebra, it must be sized to fit through the smaller posterior space, resulting in an undersized fit at the anterior end once the implant is in place. When the vertebral bodies are made to contact the opposing surfaces of the fusion implant, the curvature of the spine is straightened, producing higher stresses in adjacent levels of the spinal column and potentially leading to faster degeneration of adjacent intervertebral discs. Because some clinical problems require surgery from the posterior approach, it would be desirable to install an intervertebral fusion implant from the posterior side of the patient. It would therefore be an improvement to provide a spinal fusion implant that could recreate the natural curvature of the spine by reproducing the wedge shaped intervertebral space and concurrently allow for installation from the narrow side of the intervertebral space.